China’s air quality inspectors found problems at more than 3,000 companies in the first three months of this year, of which a large proportion were found to be falsifying data, the environment ministry said on Friday.

The Ministry of Environmental Protection said it checked more than 8,500 firms in six municipalities and provinces including Beijing and central Henan, and found that many were not implementing air pollution control measures strictly or were still violating environmental regulations.

Some companies, including a firm owned by Foxconn subsidiary FIH mobile in Hebei province’s Langfang city, tried to stop inspectors from making checks, the ministry said. Others were found to be deliberating reporting false data, it said in an online statement.

A representative for Apple supplier Foxconn said the company was not able to comment immediately in response.

China says it is winning its “war on pollution” after strengthening legislation, beefing up its monitoring capabilities and cracking down on hundreds of polluting firms, and says average air quality improved noticeably in 2016.

However, official data published last week showed that air quality was markedly worse in the first two months of the year than the same period of 2016.